1. Field of the Invention
The present invention relates to an apparatus and a method for monitoring a spot welding process and, more particularly, to a diagnosing apparatus and a method used for monitoring spot welding quality of sheet metal fabrication of automotive, appliance, aerospace industries, etc.
2. Description of Related Art
Resistance spot welding (RSW) is a process that takes advantage of the work-piece""s inherent resistance to the flow of electrical current in order to join overlapped metal sheets. RSW involves interaction of electrical, thermal, mechanical, metallurgical and surface phenomena. It has been widely used in joining metal sheets, such as those found in automotive, appliance, and aerospace industries because it requires no filler metals and has only local effects to the properties of the metal sheets being welded.
A typical sequence for RSW is presented in FIG. 6. As shown in FIG. 6, during the squeezing time, an upper electrode is pressed with a controllable static pressure against the work-pieces sitting on another lower electrode held by workers or fixtures. Then during welding time, a pre-set electric current flows through the electrodes and two-overlapped work-pieces and fusion takes place. Afterwards, in holding time, the current flow is shut off yet application of the static pressure through electrodes is sustained. In this holding period, the molten weld is allowed to consolidate as it solidifies. The pressure exerted further introduces a forging effect that enhances mechanical properties. In the last phase, the electrodes are released. The system is then ready for the next welding cycle.
According to a prior art, a more accepted method of quality monitoring for spot welding is to measure the secondary electrical resistance as shown in FIG. 7. The xe2x80x9cdynamic electrical resistancexe2x80x9d is obtained from dividing the peak or ems value of each half cycle of the measured voltage by the peak or ems value of the measured current in the corresponding half cycle. Because there are only two data points per cycle and that the voltage and current values employed in the division do not occur at the same instant, the resulted resistance record is only an approximation of the reality. Furthermore, the phase difference information between voltage and current is completely lost in the method of the prior art.
Therefore, it is desirable to provide an improved apparatus and method for monitoring the quality of spot welding process to mitigate and/or obviate the aforementioned problems.
The object of the present invention is to provide an apparatus and a method for monitoring the quality of a spot welding process in-situ, so as to diagnose the welding quality of spot welded joint in near real-time.
To achieve the objective, the apparatus for monitoring the quality of the welding process of a spot-welding system primarily comprises a current measuring device, a voltage measuring device, at least one analog to digital converter, a processing unit, and an output unit.
The current measuring device and the voltage measuring device are provided to respectively measure the electrical current and voltage signals between a pair of electrodes of the spot-welding system in every desired time interval t. The measured current and voltage signals are then converted into digital current i(t) and digital voltage v(t) by the at least one analog to digital converter. The processing unit serves for calculating an electrical impedance (Zin) between the electrodes of the spot-welding system according to the equation:             Z      in        =                  V        ⁡                  (          t          )                            I        ⁡                  (          t          )                      ,
wherein
V(t)=v(t)+jH(v(t)), and
I(t)=i(t)+jH(i(t)).
The V(t) is a complex form of the digital voltage v(t) having an imaginary part H(v(t)) obtained by Hillbert transform equation:       H    ⁡          (              v        ⁡                  (          t          )                    )        =            1              π        ⁢                  xe2x80x83                ⁢        t              *                  v        ⁡                  (          t          )                    .      
The I(t) is a complex form of the digital current i(t) having an imaginary part H(i(t)) obtained by Hillbert transform equation:       H    ⁡          (              i        ⁡                  (          t          )                    )        =            1              π        ⁢                  xe2x80x83                ⁢        t              *                  i        ⁡                  (          t          )                    .      
The electrical impedance (Zin) calculated by the processing unit is then output by the output unit, which can be a displaying device to display the electric impedance (Zin).
The apparatus of the present invention may optionally comprise a data analyzer and a memory unit, so that the data analyzer may compare a newly obtained electrical impedances (Zin) with historical data stored in the memory unit. The historical data are related to one of different quality results of a spot-welding process, such as an electrical impedance corresponding to a well-welded point, a poor-welded point, or a joint formed with expulsion, etc. The newly obtained impedance can be compared with these historical data and the conclusions of comparisons can then be displayed by the output unit or an indicator of the apparatus to reveal the quality of the welding joint. The indicator can be an LED, or any other substantial device, for lighting up when the electric impedance (Zin) coincides with one of the historical data.
Furthermore, the process unit can be made smarter for diagnosing abnormalities if so detected. Electrical impedance curves obtained from the welding processes carried out under various good and ill settings of welding parameters are obtained and stored in the memory unit automatically for comparison later. Through proper statistical or neural network pattern recognition schemes, the pattern of a newly obtained electrical impedance can be recognized. The output unit can then display the results of diagnosis, i.e., the recognized pattern of the impedance curve.
Other objectives, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.